The development of bridge transducers with long-term stabilities of the order of 0.02 percent of full scale over a temperature range of -55.degree. C. to 225.degree. C. has created a demand for high resolution .delta.R-to-digital conversion systems with commensurate long-term stabilities over these temperatures.
In a conventional Wheatstone bridge transducer, illustrated in FIG. 1, an outside quantity, such as pressure or temperature, causes a change in resistance, .delta.R, in some or all of the bridge resistors, 10, 11, 12 and 13. These resistors can be arranged in such a way that the measured quantity changes the resistances of all or some of the resistors by about the same magnitude, .delta.R, but in directions that give an additive output. For example, if the resistance of resistors 10 and 13 changes to R-.delta.R and the resistance of resistors 11 and 12 changes to R+.delta.R, the differential output voltage of such a bridge powered by a dc voltage V.sub.ref will be ##EQU1##
In pressure transducers, the full-scale of V.sub.od is usually less than one percent of V.sub.ref. Therefore, if the small output voltage of the bridge is applied directly to an N-bit analog-to-digital converter with an input range of 0 to V.sub.ref, the conversion resolution will not be better than (N-7) bits.
To obtain higher resolutions, V.sub.od in FIG. 1 must be amplified to a level comparable to the full-scale input range of the analog-to-digital converter. For example, the output of the bridge of FIG. 1 can be amplified using the single amplifier bridge balancing circuit shown in FIG. 2 or by using an instrumentation amplifier as shown in FIG. 3. However, even if modern auto-zeroing operational amplifiers ("op-amps") are used in these circuits, gain control resistors (resistors 22 and 23 in FIG. 2 and 34, 35, 36, and 37 in FIG. 3) some of whose values will normally be larger than the bridge arm resistors are required. These resistors would have to be of high quality with low temperature coefficients and low tracking temperature coefficients. Even then, they are subject to long-term drift at high temperatures and this drift affects the long-term stability of the overall measurement system.
Therefore, a need exists for a bridge transducer network producing differential output voltages with magnitudes and signs such that the network can be attached directly to a .delta.R-to-digital conversion system.
Conversion systems which use the circuits of FIGS. 2 and 3 have performances dependent on passive or active component values. Therefore, a need exists for a resistance bridge .delta.R-to-frequency conversion system with performance independent of most passive or active component parameter variations.
Furthermore, conventional bridge conversion systems which use the circuits of FIGS. 2 and 3 do not provide good noise rejection. Therefore, a need exists for a conversion system with good noise rejection.
Conventional conversion systems are not stable at high temperatures and are susceptible to component aging. Therefore, a resistance bridge .delta.R-to-digital conversion system is needed that is stable over a wide temperature range and relatively immune to component aging.
Furthermore, a need exists for a resistance bridge .delta.R-to-digital conversion system with the above properties but in integrated form.